Wavelength selective mirror systems

ABSTRACT

Polychromatic beams are separated into component colors or wavelength bands by wavelength selective filters combined with a beamsplitter. These filters are arranged on three sides of a cubic space, with a beamsplitter on the diagonal. The space may be air or a cube of transparent material, such as glass. If the beam is to be split into more than three colors, a second cube is used with additional beamsplitters. At least one filter which receives a beam split by the beamsplitter, either the filter which receives the transmitted beam or the reflected beam, must be a reflecting filter which reflects the wavelengths that are not transmitted. Preferably such filters are interference filters. Two or three reflecting filters give better energy utilization. The beamsplitters may be chromatic or achromatic. Phase retardation plates compensate to polarization effects. The invention is particularly useful with electrical signal detectors for readouts for coded marks using photoluminescent components forming the code.

Ll-LG-fd in United States Patent Travis i541 WAVELENGTH SELECTIVElVflRROR SYSTEMS Primary Examiner--David Schonberg AssistantExaminer-John W. Leonard 72 Inventor: David Neil Travis, 6O ColonialAwmeksamuel Banch walk" Road, Glenbrook, Conn. 06906 ABS TRACT 221Filed: March 25, 1970 Polychromanc beans are separated mto component PP22,575 colors or wavelength bands by wavelength selective filterscombined with a beamsplitter. These filters are Related Apphcauon Damarranged on three sides of a cubic space, with a Continuation-impart 0fbeamsplitter on the diagonal. The space may be air or March 1969, N0. acube of transparent material, such as glass. if the beam is to be splitinto more than three colors, a [52] US. Cl. .......350/171, 95/ 12.2,178/54, Second cube i used with additional 1m At 350/148, 350/157350/166 350/173 least one filter which receives a beam split by the350/174 beamsplitter, either the filter which receives the trans- [51]Int. Cl.... ..G02b 27/14, G02b 5/28 mined beam or the reflected beam,must be a fl [58] Field Search "350/169, 171, ing filter which reflectsthe wavelengths that are not 350/148 178/54 54 E; transmitted.Preferably such filters are interference fil- 95/122 ters. Two or threereflecting filters give better energy utilization. The beamsplitters maybe chromatic or [56] References cued achromatic. Phase retardationplates compensate to UNITED STATES PATENTS polarization effects. Theinvention is particularly useful with electrical signal detectors forreadouts for 3,333,053 7/1967 Back ..350/ 173 UX coded marks usingphotoluminescem components 3,497,283 2/1970 Law ........350/l48 formingthe code 3,527,523 9/1970 Travis ..350/171 3 Clains, 5 Drawing figures va t //8 l D 3 l! 1 6 WAVELENGTH SELECTIVE MIRROR SYSTEMSCROSS-REFERENCES This application is a continuation-in-part ofapplication Ser. No. 810,476, Mar. 26, I969 entitled SEPARATION OFPOLYCI-IROMA'IIC LIGHT BEAMS INTO THEIR CONSTITUENTS BY MEANS OFWAVELENGTH SELECTIVE MIRROR SYSTEM, now US. Pat. No. 3,527,523.

BACKGROUND OF THE INVENTION Resolution of beams of polychromaticradiation into component colors is very common, particularly for colorphotography and for color television. These applications requiredevelopment of good quality optical image information, that is,information developed by the separated colors must allow recombinationto produce the original image with minimal degradation of quality. In acolor television camera, for example, three images of the scene beingviewed are formed simultaneously, so that a red image of the scene isfocused on one of the camera tubes, a green image on the second, and ablue image on the third. Techniques for accomplishing this type ofchromatic resolution are highly developed and are described in theliterature, such as in pages 291-292 of Volume II, Applied Optics andOptical Engineering (Academic Press, New York, 1965), edited by R.Kingslake. Good image quality requires careful arrangement of highquality optical elements, and apparatus of this type is expensive.

SUMMARY OF THE INVENTION The present invention separates colors, usingthis term broadly to cover wavelength bands whether they are all in thevisible or whether some are in the ultraviolet and infrared, by means offilters placed on the sides or faces of a cubic space with a diagonalbeamsplitter in the space. The space may be hollow or it may be a cubeof transparent material. Either will be referred to as an opticallyhollow cube. At least one of the filters which receives either thetransmitted or primary reflected light from the diagonal beamsplittermust reflect wavelengths which it does not pass. The preferred type ofsuch a filter is an interference filter, and of course filters on allthree sides of the cubic space may be of the reflecting type. The fourthface allows entry of the polychromatic beam. It will be noted that weare dealing with those four faces of a cubic space whose normals lie ina single plane. The two other sides, i.e., top and bottom of the cube donot affect the optics of the device. The beamsplitter along the diagonalof the cube may be achromatic or chromatic. If more than three colorsare to be separated from a polychromatic beam, two cubes may be used,with an additional dichroic beam-splitter which is first encountered bythe polychromatic beam and passes a band of radiation appropriate to thefilters in the first cube and reflects the radiation appropriate for thesecond cube. This permits separation of up to six colors, but it canalso be used for separating five colors, in which case the second cubewill have only two filters and the other side may be a mirror.

The present invention is particularly useful with polychromatic beamswhose energy may be low, and where the requirements for geometric imagequality are not so high and permit some variation in path length. Veryhigh spectral purity may be obtained by the present invention whereverneeded. Energy utilization, particularly where all of the filters on thecube or cubes are reflecting, is very good, and this is of greatimportance in readers for multicomponent photoluminescent marks.Information storage and retrieval via such marks which contain mixturesof photoluminescent components, and particularly where some or all ofthe components are narrow band luminescers, is described and claimed inthe application of Freeman and I-Ialverson, Ser. No. 596,366, filed Oct.14, 1966, now U.S. Pat. No. 3,473,027, issued Oct. 14, 1969. While thereadout of photoluminescent marks is a most important field for thepresent invention, it is not limited by the nature of the source of thepolychromatic beam.

In terms of the present invention, a polychromatic beam of radiationdesignates a beam composed of photons having a number of differentenergies or wavelengths. It is not at all necessary that all wavelengthsbe represented. The polychromatic beam does not have to represent acontinuous spectrum. There may be photons in various wavelength bandswith no photons present at some intermediate wavelengths.

In any one cube of the present invention, the polychromatic beamencounters a beamsplitter section which is along the diagonal and whichis largely non-absorbing for wavelengths of interest in the separatedcolors. The incident beam is separated physically into two beams, whichmay or may not have the same spectral distribution, i.e., thebeamsplitter may be achromatic or dichroic. A separated beam then mayencounter an interference filter which transmits certain wavelengths andreflects others. This reflected radiation constitutes a beam which againencounters the initial beamsplitter and undergoes a separation into twophysically separated sub-beams, one of which may encounter anotherinterference filter. In essence, radiation reflected from interferencefilters is turned back on itself and encounters the beamsplitter sectionat least once, and sometimes more than once, before final disposition ofthe beam occurs. It is this multiple use of the beamsplitter sectionwhich allows a compact, simply constructed assembly for separating apolychromatic beam into selected component colors. Furthermore, theseseparated selected component color beams have a common optic axis in thepolychromatic beam.

It will be noted that filters, such as interference filters, areencountered by beams striking them at essentially normal incidence. Asan example, three filters can be arranged to form three vertical sidesor faces of a cube. The polychromatic beam enters through the missingvertical face of the cube, along the normal to the opposite face. Anon-absorbing beamsplitter section is placed along the diagonal of thecube, such that the beam reflected from the beamsplitter section isdirected along the normal to one of the interference filter faces. Byappropriate selection of the beamsplitter section, and of theinterference filters, an ,efficient separation of the initialpolychromatic beam into three wavelengths bands of rather high spectralpurity can be achieved.

If more than three wavelength bands or colors are to be selected out ofthe original polychromatic beam, more than one cube is used, with achromatic beamsplitter, such as an inclined dichroic beamsplitter, aheadof the assembly to effect a more gross separation into two beams, onecontaining shorter wavelengths and the other longer wavelengths of theoriginal beams. One bean encounters one cube and the other the other.With two cubes this permits selection of up to six colors. It ispossible to have more than two cubes with an additional dichroicbeamsplitter.

The diagonal beamsplitter section in the cube can be achromatic or canbe selected to exhibit different transmission to reflection ratios atdifferent wavelengths. If it is achromatic the ratio of transmission toreflection preferably is about unity, although it may vary betweenrather wide limits. If it is a chromatic beamsplitter section, thisratio preferably is near unity for one color, is higher for the secondcolor, and is lower for the third color.

The preferred interference filters sometimes may be considered astrichroic mirrors because each one passes one color and reflects othercolors on either side for a wavelength range determined by thecharacteristics of the filter. The reflected bands are not infinitelywide for a variety of reasons, some related to the intrinsiccharacteristics of optical interference phenomena particularly in thinfilms, and others related to construction and materials used inpractical filters.

In the case of achromatic beamsplitters, it is advantageous to keepwithin a range not too far from 50:50 for the ratio of transmittedintensity to reflected intensity, although 70:30 and 30:70 still areuseful. In the case of chromatic beamsplitters used with two filterswhich have high reflectivity for the third wavelength band, thetransmissivity to reflectivity ratio exhibited by the beamsplitter forthis third wavelength band should fall in the same range. Obviously thetransmissivity for one of the other wavelength bands ideally wouldapproach 100 percent, and the reflectivity for the third wavelength bandwould approach 100 percent. Selection of the chromatic behavior of thebeamsplitter can be made to complement reflectivity characteristics ofpractical interference filters.

For many dichroic beam splitters, and sometimes for achromaticbeamsplitters of the multilayer dielectric type, illumination at anangle of 45 will cause polarization of both the transmitted andreflected beams. In order to maximize the concentration of light in thedirection of the third, indirectly illuminated, side of the cube thesepolarization effects are controlled by introducing phase retardationplates between the cube and the reflective filters.

An extreme example may occur for a chromatic beam splitter with areflectance to transmittance ratio of unity for a specific wavelength.Division of the incident, unpolarized light beam may be a resolutioninto two orthogonal plane polarized light beams rather than a divisionby amplitude splitting. If these two plane polarized beams are reflectedback towards the diagonal beam splitter without any change ofpolarization, they will retrace their original paths; so that all of thelight is directed back towards the source and none towards the thirdside of the cube. This situation can be reversed by placing properlyoriented quarter wave retardation plates between the cube and the tworeflecting filters. Light beams returning toward the cube afterreflection at one of the interference filters will have passed twicethrough a quarter wave plate and hence their planes of polarization willbe rotated by relative to the planes of polarization of the same beamsafter first encountering the beamsplitter. Upon meeting the beamsplitterfor a second time these polarization rotated beams will behave in amanner opposite to that at the original encounter; e.g. if firstreflected they will be transmitted and vice versa. In this extremeexample the addition of quarter wave plates causes all the light of thespecific wavelength to be directed toward the third side of the cube andnone is lost in the direction of the source.

In the more general case where the diagonal beamsplitter produces onlypartial polarization, not all of the available light of the specificwavelength can be concentrated on the third side of the cube, but theinclusion of retardation plates gives optimum efi'iciency.

Phase retardation plates suitable for use in the present invention neednot be of high optical quality since no geometric imaging is required.Plates made from a birefrigent plastic such as cellophane may be used,and are readily prepared and inexpensive. A sample of plastic having apreferred phase retardation is selected by trial.

The polychromatic beam in the present invention need not be collimated,although, of course, it can be. It is an advantage that the particularnature of the polychromatic beam is, therefore, not sharply critical andthat compromises which will give good energy utilization with adequateresolution can be chosen for many uses. This is a very differentsituation from prior art proposals for television cameras, photographicwork and the like, where very sharp geometric resolution is essential.The present invention with its efficient energy utilization cannot beused for such devices where sharp imaging is essential. This is furtherevidence that emphasizes that the present invention operates underdifferent optical requirements than those used in color photography andcolor television.

Reference has been made above to the fact that the cube may be hollow orthat it may be made partly or wholly of transparent material. In eachcase the radiant energy is able to pass, and this will be referred tomore generally throughout the remainder of the specification as anoptically hollow cube.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustrationof separating three colors by means of a single optically hollow cube,with a converging incident polychromatic beam;

FIG. 2 is a similar diagram of a modified cube with a collimatedincident beam;

FIG. 3 is a diagram of a cube tracing one of the colors and showingmultiple reflections;

FIG. 4 illustrates separating six colors with two cubes and an auxiliarybeamsplitter;

FIG. 5 illustrates the separation of three colors using a polarizationsplitting dichroic mirror.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates indiagrammatic form an optically hollow cube. This also illustrates aconverging beam formed by the lens (8). A light beam containing thethree wavelengths i\,, 1 and A encounters an achromatic beam splitter 9at 45 incidence, in this particular illustration a 50:50 splitter. Onehalf of the light is transmitted toward interference filter 1, withintensity which may for convenience be designated 1/2 A, U2 1/2 ASimilarly one half of the original beam is reflected toward filter 2 andis again designated l/2 A, H2 A H2 A The transmitted beam impinges uponfilter l where the component l/2)\ is transmitted through the detector DThe remainder (H2 l/2 A is reflected back toward the beam splitter 9,filter 1 being illustrated as a good reflector for M and A In a similarway, that half of the original beam reflected by 9 and containing l/2)tl/ZA l/2 is incident upon filter 2 where again l/2)t is transmitted todetector D and the remainder (l/2lt l/2A reflected back towards 9. Thelight beam l/2)\ l/2A coming from filter 1 is partially reflected by 9towards filter 3 with intensity I/4)t l/4A whilst the beam l/2)t l/2Acoming from filter 2 is partially transmitted by 9 in the direction offilter 3 and with intensity l/4)\ H4) Thus, in all, there impinges uponfilter 3 a total intensity expressed by (l/4)t l/41\ I/ZA The l/2X ispassed through filter 3 to detector D while (l/4)t -l- H412) isreflected back towards beam splitter 9 for a third encounter. A smallfraction ofthe light may reach filtersl and 2 again after variousreflections and will ultimately reinforce those components l/ZM and l/ZMwhich reach the filters after only one encounter with (9). Thesesecondary contributions to the detectors D and D will be out of focus atthe detectors since they have travelled along a greater path length thanthe primary contributions to these detectors. Where the beam resultsfrom a spot in a reader for a photoluminescent code, this defocusing isof little significance; but of course if the beam were to be going totelevision cameras or photographic negatives, it would be uselessbecause there would be blurring, which, as has been pointed out above,is a further indication of the different optical principle under whichthe present invention operates.

FIG. 3 illustrates in symbolic form what happens to one color,designated by A of a polychromic beam impinging on a color separatorassembly of the present invention. For convenience beam splitter 9 isselected to transmit one half the intensity of X and to reflect onehalf. This is indicated symbolically by making the transmitted andreflected beams one half as wide as the incident beam in FIG. 3. Theseinitially transmitted and reflected beams are shown striking filters land 2 respectively. For convenience both filters l and 2 are depicted asreflecting A completely. The transmitted portion of A is turned back onitself by filter l, and on striking beamsplitter 9 one half of it isreflected down to filter 3 which is depicted as transmitting color Acompletely. This beam, representing one fourth of the initial incomingbeam, is designated by hatching in FIG. 3. Similarly, the portion of theincoming beam which was reflected to filter 2 is turned around onitself, impinges on beamsplitter 9 again, and one half is transmitted tofilter 3 and passes through. us, out of the initial beam, one half ofthe intensity of A; is depicted by the inked in lines as transmitted byfilter 3, while one half of it is lost. If the transmissivity andreflectivity of beamsplitter 9 for A are designated by t and rrespectively, and the reflectivities of filters l and 2 for A aredesignated by R, then the fraction of the initial intensity of A whichimpinges on filter 3 is given by the expression 2 X R X r X z. The onlyangles involved in construction are and 45, and filters l, 2 and 3 alloperate at normal incidence.

Beamsplitter 9 in FIG. 3 can be either chromatic or achromatic. Ifchromatic, transmission should be larger than reflection for wavelengthspassed by filter I, and reflection should be larger than transmissionfor wavelengths passed by filter 2. It is preferable that reflectivityand transmissivity be approximately equal for wavelengths transmitted byfilter 3.

As FIG. 3 is merely a diagram of the ray paths for M, the otherelements, such as detectors, lenses and the like, are omitted. Also,FIG. 3 shows a collimated beam coming in, which somewhat simplifies theillustration of the rays. The rays are shown turned with a curve, but ofcourse in an actual device the reflections would be at precise angles.

FIG. 5 is a diagram similar to FIG. 3 but illustrates a chromatic beamsplitter 15 which resolves a into two plane polarized beams of equalintensity. P represents the polarized light beam whose electric vectorvibrates in a plane parallel to that of the figure. Pl. represents thebeam whose electric vector vibrates in a plane perpendicular to that ofthe figure. Quarter wave retardation plates 17 and 16 for wavelength Aare located between the cube and filters for A, and A, 18 and 19.Parallel polarized light of wavelength A; is transmitted by 15 andpasses through the quarter wave plate 17 becoming circularly polarized PAfter reflection at the filter 18 it returns through the quarter waveretardation plate (17) and becomes plane polarized again, but in a planerotated by 90 from that of the beam initially transmitted by the beamsplitter 15. In other words, parallel polarized light is converted toperpendicularly polarized light by the double passage through thequarter wave plate. Perpendicularly polarized light at the secondencounter with the chromatic beam splitter 15 is titally reflectedtowards the filter 20. Similarly the perpendicularly polarized beaminitially reflected by the chromatic beam splitter 15 is converted intoparallel polarized light after passing twice through the quarter waveplate 16 and reflection at the filter 19. Upon meeting chromatic beamsplitter 15 again it is totally transmitted towards the filter 20. Inthis specific example, all the radiation of wavelength A is directedtowards the third filter l9 and none is lost in While maximum effect canbe obtained by a dichroic mirror that splits the parallel andperpendicular components of beam a exactly, a less than perfectsplitting gives effective results, and if the reflected and transmittedbeams are elliptically polarized, retardation plates are chosen whichmaximize the intensity of the wavelengths of interest at the detectors.

FIG. 2 is a diagram similar to FIG. 1 but shows a beam collimated bylens 8, with additional lenses 4, 5 and 6 which reimage the mark withradiation transmitted by filters l, 2 and 3 onto their respectivedetectors D,, D D The optical arrangement in FIG. 2 uses a collimatedbeam within the cube, as contrasted with a converging beam in FIG. 1.This minimizes the blurring of images caused by optical path lengthvariations within the cube for different rays reaching the samedetector. Hence somewhat smaller detectors can be used in FIG. 2 thanwould be appropriate for FIG. 1 for the same size marks. With somedetectors this increases speed of response.

FIG. 2 also illustrates a modified form of optically hollow cube whichis solid although transparent, for example a glass cube. The form of thebeamsplitter here is somewhat different than in FIG. 1 and so it isgiven a different reference numeral, 14. On the other hand thebeamsplitter arrangement of FIG. 1 is also useable in FIG. 2.

FIG. 4 illustrates the situation with six colors, using two opticallyhollow cubes. The one to the right is the same as shown in FIG. 2 andthe parts bear the same reference numerals. However, there is anadditional beamsplitter 7 which reflects part of the beam into a secondoptically hollow cube which is similar to the first one but is providedwith a different beamsplitter l and filters ll, 12 and 13 for colors AA, and A The detectors which receive these colors from the filters aredesignated D D and D respectively.

As has been stated above, the cubes in FIGS. 1 to 4 are opticallyhollow. It will also be noted that the drawings have shown sectionsthrough the cubes which, of course, in section are squares with one sideopen. The top and bottom of the cube do not function in the lighttransmission system.

In the drawings, such as FIGS. 1 to 4, the filters may all be reflectingfilters, but it will be apparent from a consideration of FIG. 3 thatactually only one of the filters must of necessity be reflecting. Thisis the filter which receives either the transmitted beam through thebeamsplitter 9 as shown in FIG. 3 or it could be filter 2, which wouldreceive the primary reflected beam from the beamsplitter 9. Theoperation is the same, but, for example, in FIG. 3 if filters 2 and 3were absorption filters only half of the available energy in A namelythe hatched portion to the right of the figure, would be received. Ingeneral, the question of whether more than the necessary one reflectingfilter should also be reflecting can be chosen in accordance with energyand other requirements, which shows that the invention is capable of agreat deal of flexibility.

Where chromatic bearnsplitters are used, as has been stated, sometimesthe chromatic selection is sufliciently marked that it is not necessaryto increase the energy by the multiplereflections. In such a case filter3 would not have to be reflecting. However, reflecting filters arereasonably economical and it is therefore preferable though notessential to use them throughout even with chromatic beam splitters.

The descriptions of the drawings have been in terms of the maximumnumber of colors to be separated. It should be obvious that if a beamhas less than the maximum number, the missing color will not bedetected. When dealing with multiple cubes, it is not necessary that thefull number of separable colors be provided for. For example, if a beamhas five colors instead of six, one of the sides of one of the cubes canbe a plane mirror.

The wavelengths are referred to as A A and A these may in general beprimary colors such as red, yellow and blue, but more conveniently ininformation storage and retrieval, particularly with photoluminescentmarks and readouts, are sharply chosen to match particular components ofinterest some of which may have wavelengths outside of the visibleregion.

The system of multiple cubes for more than three wavelengths isparticularly useful with rare earth lurninescers with interferencefilters sharply selective for luminescence from components such asdisclosed in US. Pat. No. 3,473,027, supra at:

Luminescence-wavelength Rare Earth As can be seen from the comparativelynarrow spacing between lines, sharper interference filters are requiredthan would be used for resolution into red, blue and green for primarycolor separation in photography or television.

A system for splitting white light into red, blue and green componentsis described in US. Pat. No. 3,497,283, Law, COLOR SELECTION POLARIZINGBEAM SPLITTER, Feb. 24, 1970.

With coded luminescent systems, the reliable identification of at leastsix of these components is desirable, and by plural cube systems allnine can be resolved without undue loss of optical energy, Interferencefilters with a pass-band of 200 Angstroms permit such resolution.

Iclaim:

1. A system for separating different wavelength bands from apolychromatic beam of radiation having wavelengths which obey opticallaws which comprises,

a. an optically hollow cube having a diagonal plane chromaticbeamsplitter, the two edges thereof coinciding with edges separatingfour faces of the cube,

b. one of said faces being open to an incident polychromatic beam andreflecting interference filters on at least two of the other threefaces, each said interference filter passing a different wavelength bandof radiation, and reflecting substantially all others, each saidwavelength band being about 200 angstroms, and hence being IOGIDSsharply selective for the wavelengths of specific rare earthluminescers, whereby the incoming polychromatic beam is split by thebeamsplitter, part being reflected to a first face and part transmittedto a second face of the cube, and each interference filter on the firstand second faces reflects a selected portion of the radiation backtoward the beamsplitter which transmits and reflects, respectively, atleast part of the selected portion of the radiation to the third face ofthe cube, whereby one wavelength component of the incoming polychromaticbeam is largely reflected to the first face where it is passed by theinterference filter at that face, another wavelength component islargely transmitted to the second face where it is passed by theinterference filter at that face, and a third wavelength component ispartially reflected and partially transmitted in substantiallycomparable amounts toward the first and second faces respectively, wherethe interference filters reflect the component back toward thebeamsplitter,

. a phase retardation plate being placed in the radiation path betweenthe beamsplitter and the first cube face and between the beamsplitterand the second cube face so that on passing through the plate twice thethird wavelength component has its polarization altered to reverse itstransmission and reflection characteristics on a second encounter withthe beamsplitter, thereby causing the third wavelength component of theinitial polychromatic beam to be diverted to the third face of the cube,

. the cube being oriented so that the optical axis of the polychromaticbeam passing through the beamsplitter strikes the open face of the cubeat substantially normal incidence.

2. A system according to claim 1 in which said filters are on threesides and all of the filters on the three sides of the cube areinterference filters.

3. A system according to claim 1 for separating more than three but notmore than six colors from a polychromatic beam, in which a secondoptically hollow cube is used and a chromatic beamsplitter is positionedto reflect one broad band of radiation into one cube and transmitanother and different broad band of radiation into the other cube, thetotal number of interference filters on the faces of the two cubescorresponding to the number of colors to be separated.

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1. A system for separating different wavelength bands from apolychromatic beam of radiation having wavelengths which obey opticallaws which comprises, a. an optically hollow cube having a diagonalplane chromatic beamsplitter, the two edges thereof coinciding withedges separating four faces of the cube, b. one of said faces being opento an incident polychromatic beam and reflecting interference filters onat least two of the other three faces, each said interference filterpassing a different wavelength band of radiation, and reflectingsubstantially all others, each said wavelength band being about 200angstroms, and hence being sharply selective for the wavelengths ofspecific rare earth luminescers, whereby the incoming polychromatic beamis split by the beamsplitter, part being reflected to a first face andpart transmitted to a second face of the cube, and each interferencefilter on the first and second faces reflects a selected portion of theradiation back toward the beamsplitter which transmits and reflects,respectively, at least part of the selected portion of the radiation tothe third face of the cube, whereby one wavelength component of theincoming polychromatic beam is largely reflected to the first face whereit is passed by the interference filter at that face, another wavelengthcomponent is largely transmitted to the second face where it is passedby the interference filter at that face, and a third wavelengthcomponent is partially reflected and partially transmitted insubstantially comparable amounts toward the first and second facesrespectively, where the interference filters reflect the component backtoward the beamsplitter, c. a phase retardation plate being placed inthe radiation path between the beamsplitter and the first cube face andbetween the beamsplitter and the second cube face so that on passingthrough the plate twice the third wavelength component has itspolarization altered to reverse its transmission and reflectioncharacteristics on a second encounter with the beamsplitter, therebycausing the third wavelength component of the initial polychromatic beamto be diverted to the third face of the cube, d. the cube being orientedso that the optical axis of the polychromatic beam passing through thebeamsplitter strikes the open face of the cube at substantially normalincidence.
 2. A system according to claim 1 in which said filters are onthree sides and all of the filters on the three sides of the cube areinterference filters.
 3. A system according to claim 1 for separatingmore than three but not more than six colors from a polychromatic beam,in which a second optically hollow cube is used and a chromaticbeamsplitter is positioned to reflect one broad band of radiation intoone cube and transmit another and different broad band of radiation intothe other cube, the total number of interference filters on the faces ofthe two cubes corresponding to the number of colors to be separated.